The human body is made of 60 trillion cells. Cancer is a pathological state that threatens an individual's life by the cells being genetically changed and obtaining a potency of uncontrolled autonomous growth. A process of the development of cancer cells from normal cells have been elucidated by experiments on chemical carcinogenesis and it has been found that tumor development occurs as the result of the accumulation of genetic alterations.
From a standpoint of cell biological study of cellular alteration, the process of carcinogenesis is discussed as different stages of genetic alterations, i.e., initiation, promotion and progression. Initiation is a process in which a fixed change in a DNA sequence is given in a target cell. Promotion is a process in which the growth of an initiated target cell is selectively affected by various factors. However, the target cell still remains at a stage of a benign tumor. Progression is a process in which a conversion of a benign tumor cell into a malignant one proceeds by an acquisition of genetic changes which enable the tumor cell to exhibit more aggressive properties manifested by rapid growth, invasiveness and metastatic ability.
It is known that genetic changes are provoked by chemical carcinogens, radiation and reactive oxygens. Among them, reactive oxygens produced by inflammatory cells such as neutrophils may be a primary factor in the development of up to one-third of all cancer cells (Ames et al, Proc. Natl. Acad. Sci. USA 90; 7915–7922 1993).
Reactive oxygen species have long been suspect as agents which induce a genetic mutation. Reactive oxygen species include superoxides (O2−), hydrogen-peroxide (H2O2), hydroxy radicals (OH−) and singlet oxygen (1O2). About 1 to 3% of oxygen inspired into the body by respiration becomes reactive oxygen during reduction to water. Reactive oxygen species have a potent oxidative activity and they are toxic to life. Therefore, nature developed by evolution a system which scavenges reactive oxygen for the purpose of detoxication of reactive oxygen by inducing/absorbing scavenging enzymes/antioxidants. Accordingly, as far as in the range of normal respiration, reactive oxygens generated may be detoxicated. However, various damages will occur when the amount of generated reactive oxygen exceeds the amount of detoxication capacity of reactive oxygen. Generation of reactive oxygens takes place at a site of inflammation in the body. Activated leucocytes and other phagocytic cells continuously produce vast amounts of reactive oxygen which cleave and modify DNA, oxidize lipids and sugars, denature proteins and inactivate enzymes resulting in damage to DNA and biomembranes. The present inventors developed an animal model which evaluates the effects of reactive oxygen to progression of cancer cells (Okada, F. et al, Br J Cancer. 66:635–639 1992).
The present inventors established a tumor cell clone (BMT-11cl-9) by cloning from a tumor developed by subcutaneous injection of a chemical carcinogen, methyl-cholanthrene, in a C57BL/6 mouse. BMT-11cl-9 cells were exposed in vitro to quercetin, a mutagen, at a concentration of 55 μm and several number of cloned cell lines were established. QR-32 cell line was one of them. QR-32 tumor cells do not grow in syngeneic C57BL/c mice when limited numbers of cells are injected intravenously or subcutaneously. However, a lethal growth occurs when QR-32 tumor cells are injected in immuno-suppressed mice. Accordingly it was found that QR-32 cell line was tumorigenic and that QR-32 was equivalent to a dormant tumor or a benign tumor. However, those tumor cells grew and formed a tumor mass when co-transplanted with a gelatin sponge. Then, the tumor mass was excised and a tumor cell suspension was made to establish culture cell lines from the tumor. Subcutaneous transplantation of the cells of these cell lines made a tumor mass at the injection site and intravenous injection of the cells of these cell lines produced lung metastasis. The results showed that the cells of the original QR-32 cell line which was not able to grow in vivo changed to be more malignant by co-implantation of the original cells with gelatin as shown by tumorgenicity and metastatic malignancy of these cell lines. As for mechanisms of malignancy, it was thought that because the gelatin-sponge was foreign to mice, inflammatory cells such as polymorpholeucocytes accumulated at the site of gelatin-sponge insertion and generated reactive oxygens. The reactive oxygens induced DNA mutation resulting in accomplishing malignant transformation of benign tumor cell line QR-32.
As for methods of treatment of tumors, there are surgical therapy, radiation therapy, chemotherapy and immunotherapy. The practice of cancer treatment is conducted by one of/or combining these treatment methods. Tumor masses which can be resected surgically are, as a general rule, treated by surgical operation. Tumor masses which cannot be treated by surgical operation, however, are treated by one or combinations of the other therapy methods except surgery. By the way, it is thought that progression of tumor cells, such as the loss of hormone-dependency (prostate tumor, breast tumor), acquisition of resistance to anti-tumor agents, metastatic ability and more rapid growth rate occur during the process of cancer treatment. Therefore, it is important to obstruct malignant transformation of tumor cells which occur during the process of cancer treatment for the purpose of successful tumor therapy.
Superoxide dismutase (hereinafter referred to as SOD) is an antioxidant enzyme that catalyzes the conversion of a super-oxide to hydrogen peroxide and is known as the enzyme that plays a central role in protecting from oxidative damages. Postarie and Eric invented a preparation of SOD molecules coated with lipids or proteins (hereinafter referred to as coated SOD preparation). They found that oral or parenteral administration of the preparation to animals elevated activity of SOD in the body compared with a preparation of SOD alone (U.S. Pat. No. 6,045,809).